1. Field of the Invention
The present invention relates to improved apparatus for making electrical connection to an electrode of a capacitor, and more specifically, to improved apparatus for making electrical connection to an electrode of a ceramic capacitor so that electrical stress concentrations at the edge of the electrode are minimized and so that the possibility of damage to the brittle ceramic substrate of the capacitor is minimized. Additionally, the present invention relates to apparatus of the type described above for making electrical connection to both electrodes of a ceramic capacitor, to a subassembly of capacitors using the electrical connection apparatus, to an assembly of capacitors using the subassembly, to a capacitor rod which includes the capacitor assembly, and to a capacitive voltage divider using the capacitor rod.
2. Prior Art
Ceramic capacitors are well known as is their use in conjunction with high-voltage-circuits. Typically, ceramic capacitors include a ceramic substrate, usually made of barium titanate, which has a very high dielectric constant (on the order of several hundred to several thousand with respect to air) and a very high dielectic strength (on the order ot 50 to 300 volts/mil). Typically, the substrates take the form of thin, round or rectangular disks having two opposed major surface areas. Capacitor electrodes are typically formed by applying to the two surface areas of the substrate a silver dispersion which is fused to the ceramic disk at high temperature. Because of the high dielectric constant and strength of the barium titanate substrates, ceramic capacitors exhibit very large capacitances and very high withstand voltages for their size. Ceramic capacitors do exhibit some voltage and thermal sensitivity. That is, the amount of capacitance exhibited by a ceramic capacitor and its withstand voltage are, to some extent, dependent upon the voltage applied to the capacitor and the temperature thereof. Because barium titanate exhibits piezoelectric properties, the substrate may expand or contract (electrostriction) when the capacitor is discharged or charged. Piezoelectric or thermal expansion or contraction of the substrate, and the brittleness of barium titanate, lead to a requirement that ceramic capacitors be utilized in packages or housings which minimize the effects of external forces applied to the capacitor and which permit the substrates to expand or contract due to electrostriction and thermal effects without cracking or breaking.
High electrical stress concentrations may occur at or near the outer edge of the silver electrodes of ceramic capacitors. These high stress concentrations may break down the dielectric surrounding the capacitors, causing flashover between the electrodes. A stack of series-connected ceramic capacitors may be formed to produce a capacitor subassembly. The high electrical stress concentrations at or near the edges of the capacitors may cause flashover between adjacent capacitors. Where ceramic capacitors are used at high voltage, if the silver electrodes thereof terminate short of the periphery of the ceramic substrate, significant stress concentrations may be present within the substrate itself between the edge of the electrode and the edge of the substrate. If adjacent capacitors in a subassembly are not accurately aligned so that their silver electrodes do not have their edges accurately aligned, flux lines emanating from an overhanging electrode crowd around the edge of an underhanging electrode as they enter the ceramic thereunder. This crowding is evidenced by increased stress concentration in the area surrounding the capacitors. If the ceramic substrates of adjacent capacitors are precisely aligned, but the peripheral edges of the silver electrodes thereon are not, undesirable high electric stress will occur in the gap between an overhanging coating and the ceramic substrate supporting the underhanging coating. Both effects may break down any air or dielectric in this gap, leading to failure of the capacitor subassembly.
The prior art exhibits a variety of facilities for attempting to reduce the flashover or electrical stress breakdown of ceramic capacitor subassemblies, as well as for attempting to reduce problems generated by the application of external forces to the capacitor subassembly by expansion and contraction of the stack due to both thermal variations and electrostriction.
Nakata in U.S. Pat. No. 3,586,934 discloses a high-voltage ceramic capacitor comprising a stack of ceramic capacitors. The facing electrodes of adjacent capacitors of the stack are bonded together, as with an epoxy cement filled with silver (or with silver solder) therebetween. The outer periphery of the bonded capacitor stack is then centerlessly ground until the silver electrodes and the barium titanate substrates have their peripheral edges precisely aligned with respect to each other about the entire periphery of the capacitor stack. A thin layer of a dielectric material is then applied to the periphery of the stack. The capacitor stack may be included in a final assembly by urging against the end electrodes metal members, one of which may be spring loaded. Nakata's centerless grinding prevents any of the silver electrodes from overhanging or underhanging and is performed in such a way that particles of the silver electrodes and of the ceramic substrates do not adhere to the periphery of the stack. Thus, there are no sharp edges, sharp points, or overhanging or underhanging conductive members; concentrated high electrical stress at the periphery of the stack is minimized. Further, the thin dielectric coating on the periphery of the stack surpresses field emissions and microdischarge-initiated breakdowns from the edge of the silver electrodes. According to Nakata, the dielectric coating must have a high resistivity, preferably in the neighborhood of 10.sup.8 ohms-centimeter.
The capacitor stack of the Nakata patent is rather expensive and complicated to manufacture, involving as it does both centerless grinding and the bonding together of numerous small ceramic capacitors into a stack. Further, while the centerless grinding operation, in combination with the use of the thin dielectric coating on the periphery of the stack, may reduce the possibility of flashover or electrical stress breakdown of the capacitor stack, the possibility of such events occurring still remains. Further, although the stack as a whole may expand and contract axially due to the spring loading of the metal member, radial expansion and contraction of the stack can lead to problems. Specifically, if one capacitor in the stack expands or contracts at a different rate than an adjacent capacitor in the stack, the bond between the facing silver electrodes of these two capacitors may be broken, cracked, or otherwise degraded.
Mankoff and Nakata in U.S. Pat. No. 3,325,708 disclose a high-voltage capacitor assembly similar to the above-described Nakata patent. Each capacitor in the stack is surrounded with a toroidal insulator which covers the periphery of each ceramic substrate and the edge of the silver electrodes thereon, but which exposes through its central hole the central portion of the silver electrode. Each insulator which is preferably high dielectric strength epoxy, is bonded to its capacitor with, for example, a thin varnish layer. A metallic button made of steel, which may be cadmium plated, is interposed between adjacent capacitors to electrically interconnect their facing silver electrodes. The entire stack is surrounded by an insulative tube to which are threadingly attached end terminals. The end terminals each engage a conductive end button similar to the buttons between adjacent capacitors and push these buttons against the stack. One of the end buttons may be spring-loaded. An objective of the Mankoff and Nakata patent is to eliminate failure of the bonded joint between individual surrounding capacitors, as discussed above with reference to the '934 patent. To this end, the conductive buttons are not bonded to the silver electrodes of individual capacitors, but are merely held in place by the compressive axial force exerted on the capacitor stack by the end buttons. The purpose of the high dielectric strength insulators on the peripheries of the ceramic substrates of the individual capacitors and on the edge on the silver electrodes thereof is to prevent high electrical stress concentrations from ionizing air surrounding the individual capacitors to thereby prevent flashover along the periphery of the capacitor stack. The use of the varnish to bond each insulator to its capacitor is said, by Mankoff and Nakata, to prevent air from being trapped at the interface between each insulator and its substrate and silver electrodes. In short, high electrical stress, which occurs at the edge of the silver coatings, is forced to occur within the high dielectric strength insulator which will not break down as a result thereof. Further, because of the spring-loading of one of the conductive end buttons, the capacitors in the stack can axially expand and contract freely. Moreover, unequal radial expansion or contraction of individual capacitors, according to Mankoff and Nakata, simply results in a slight relative sliding between the conductive buttons and the silver electrodes against which each bears.
The capacitor stack of the Mankoff and Nakata patent is rather complicated and expensive to manufacture. Although it apparently alleviates some of the problems previously discussed above relative to the capacitor stack of the '934 patent, it should be noted that the Mankoff and Nakata capacitor stack involves a rather crucial step, mainly, applying each insulator to its capacitor with varnish or other adhesive so as to insure that no air is trapped between the interface of the insulator and both the ceramic substrate and the edges of the silver electrodes.
Nakata, U.S. Pat. No. 3,670,222, discloses a capacitor assembly comprising a stack of individual ceramic capacitors. The structure of this Nakata patent appears to involve a combination of the structures disclosed by the previous two patents. Dutta, U.S. Pat. No. 3,731,130, and Folta, U.S. Pat. No. 3,040,385, both relate to capacitor assemblies which include, in part, heat shrinkable outer plastic sleeves for holding the capacitor assembly together. Neither of these patents appear to disclose ceramic capacitors of the type described herein. Leeds, et al., U.S. Pat. No. 4,840,670, Mashikian, et al., U.S. Pat. No. 3,673,305, Haefely, U.S. Pat. No. 2,086,078, Brafman, U.S. Pat. No. 2,777,976, and Devins, et al., U.S. Pat. No. 3,522,495, all disclose various forms of capacitor assemblies intended for use in high-voltage environments in which each assembly is ultimately contained within a ceramic or cured resin housing. Some of these patents, for example Haefely and Devins, disclose filling the housing with a high dielectric strength medium, such as a insulating oil or SF.sub.6 gas. Further, some of these patents, typified by Leeds, show the need to take into consideration the fact that ceramic capacitors may expand or contract and, therefore, utilize spring-loaded terminal assemblies similar to those described above.
A general object of the present invention, then, is to provide both improved apparatus for making electrical connection to a ceramic capacitor and an improved assembly comprising a stack of capacitors utilizing the improved electrical connection apparatus so that a capacitor assembly which is inexpensive and simple to manufacture, reliable in operation, and electrically and mechanically robust is provided.